1. Field of the Invention
The present invention relates to an image coding apparatus and image coding method, image decoding apparatus, recording medium and image processing apparatus and image transmitting method, and more particularly relates to the image coding apparatus and image coding method, image decoding apparatus, recording medium and image processing apparatus and image transmitting method capable of thinning-out (subsampling) and compression encoding an image in such a manner that a decoded image is almost identical to a source image.
2. Description of the Related Art
Conventionally, various methods have been proposed as methods for compressing images. One such method employs the original image data (i.e., the image to be encoded) as an image of a "first hierarchy" (uppermost hierarchy). An image of a second hierarchy and an image of a third hierarchy are formed by reducing the number of pixels (i.e., resolution is successively lowered).
In accordance with a conventional hierarchical coding system, images of plural hierarchies are transmitted from a transmitting device to a receiving device. At the receiving device, images of the respective hierarchies may be displayed (e.g., on a monitor) in response to each of these images of the plural hierarchies.
Moreover, in accordance with a conventional hierarchical decoding system, an error correction process operation is carried out for data about the image in the lowermost hierarchy (namely, the image with the lowest resolution) but not for the images of the other higher hierarchies. As a result, when an error happens to occur, no error correction can be made as to the images of the hierarchies other than the lowermost hierarchy. As a result, under any error condition, only the data of the image of the lowermost hierarchy can be acquired and corrected. Images of the hierarchies higher than the lowermost hierarchy do not have data for error recovery and,t thus, may be obtained only by way of, for example, an interpolation process operation from the data about the image of the lowermost hierarchy on the receiver end. Therefore, the robustness characteristic with respect to the error can be improved in accordance with the hierarchical coding system.
FIG. 36 illustrates one example of a conventional image coding apparatus for performing the above-described hierarchical coding operation. Image data to be coded is supplied as data of a first hierarchy (uppermost hierarchy) to a thinning (i.e., sub-sampling) unit 11 and a signal processing unit 501.
In the thinning unit 11, the pixel quantity of the image data of the first hierarchy is thinned, so that image data of a second hierarchy (lower than the first hierarchy by one hierarchy) is formed, and the formed image data is supplied to a thinning unit 12 and the signal processing unit 501. In the thinning unit 12, the pixel quantity of the image data of the second hierarchy is thinned, so that image data of a third hierarchy (further lower than the second hierarchy by one hierarchy) is formed, and this image data is supplied to the signal processing unit 501.
In the signal processing unit 501, an error correction process and other necessary signal processing operations are carried out with respect to the image data of the first hierarchy, the second hierarchy and the third hierarchy. Thereafter, the signal processed image data are multiplexed and the multiplexed image data is outputted as the coded data. It should be noted that in the signal processing unit 501, the image data of the third hierarchy is processed by executing the stronger error correction than that of the data of other hierarchies.
FIG. 37 illustrates an example embodiment of a conventional image decoding apparatus for hierarchically decoding the coded data outputted from the image coding apparatus of FIG. 36.
In a signal processing unit 601, the coded data is separated into the coded image data of the first hierarchy, the second hierarchy and the third hierarchy. In addition, the signal processing unit 601, the error correction process and other necessary process operations are carried out with respect to the separated image data. The image data of the first hierarchy is directly outputted as a decoded image of a first hierarchy. The image data of the second hierarchy is supplied to an interpolating unit 602. The interpolating unit 602 executes the interpolating process with respect to the image data of the second hierarchy so as to produce image data having the same pixel number as that of the image data of the first hierarchy (higher than that of the second hierarchy by one hierarchy). Then, this produced image data is outputted as a decoded image of a first hierarchy.
The image data of the third hierarchy is supplied to another interpolating unit 603. The interpolating unit 603 executes the interpolating process operation for the image data of the third hierarchy to thereby produce image data of the same pixel number as that of the image data of the second hierarchy (higher than this third hierarchy by one hierarchy), namely a decoded image of a second hierarchy. The decoded image of the second hierarchy is outputted to another interpolating unit 604. The interpolating unit 604 executes a similar interpolation process to that of the interpolating unit 602 on the output of the interpolating unit 603, so that image data having the same pixel number as that of the image data of the first hierarchy (further higher than the second hierarchy by one hierarchy) is produced, and then this image data is outputted as a decoded image of the first hierarchy.
As a result, even when the image data of the first hierarchy could not be obtained for some reason, the decoded image of the first hierarchy may be obtained from the image data of the second hierarchy in the image decoding apparatus. Similarly, even when the image data of the first hierarchy and the image data of the second hierarchy could not be obtained due to same reasons, it is possible to obtain the decoded images of the first hierarchy and the second hierarchy from the image data of the third hierarchy.
However, the image quality of the decoded image of the upper hierarchy, being interpolated from the image(s) of the lower hierarchies, is considerably deteriorated.
It would be desirable to provide a system capable of obtaining a decoded image with a high image quality even if only obtained from coded data of a lowermost hierarchy.